by Oren Dorell, USA TODAY

by Oren Dorell, USA TODAY

Reports that Syria's Assad regime launched what appeared to be a massive chemical attack Wednesday morning, killing hundreds of civilians, have prompted renewed calls for U.S. action.

The attack on the eastern suburbs of Damascus came just after the arrival of United Nations weapons inspectors investigating sites where the United States and France have said the regime launched earlier, smaller chemical attacks. Wednesday's attack came a year and a day after President Obama declared that Bashar Assad's use of chemical weapons would cross a "red line" that would prompt the USA to take action.

Rep. Eliot Engel, the ranking Democrat on the House Foreign Affairs Committee, said that if the reports are true, the United States should follow through on the threat.

"If reports are credible that the Assad regime has used chemical weapons resulting in the estimated deaths of hundreds of civilians, then clearly a red line has been crossed again," Engel said. "The U.S. has two options: Continue to largely stand on the sidelines as the regime slaughters its own people, or tip the balance of power against a brutal dictator by degrading its ability to attack civilians. If we are to salvage what remains of our credibility in the region, we must act soon."

A spokesman for the Syrian government denied the reports.

"All what has been said is ridiculous and naive, unscientific, illogical and subjective," said Information Minister Omran al-Zoubi, speaking to Syrian state television.

The state-run SANA news agency said there was no truth to the allegations "whatsoever" and said the reports were meant to divert the U.N. inspectors from their work.

Other observers, however, say there is evidence to the contrary: dozens of videos from the scene showing hundreds of victims, including women and children, in distressed positions and wrapped in funeral shrouds with no apparent injury.

Hamish de Bretton-Gordon, a former commander of the chemical biological and nuclear counter terrorism unit at the United Kingdom's Ministry of Defense, said he viewed 75 videos posted and concluded that "these people didn't die of conventional weapons."

"I think this is the most significant day in the war thus far," Bretton-Gordon said. "One is struggling to find an alternative to chemical weapons" to explain the images.

Other experts said the evidence so far is not conclusive.

Similar symptoms could result from a riot control agent such as tear gas used in confined spaces, a toxic industrial chemical new to warfare, or a large conventional explosion that sucks air out of a large underground shelter and then blows in smoke and debris, said Dan Kaszeta, managing director of U.K.-based security consultancy Strongpoint Security, who worked 20 years on chemical biological and nuclear defense in the U.S. government and military.

None of those alternatives "is compelling," yet they cannot be ruled out, Kaszeta said. "Until someone actually comes up with a sample of the causative agent (suspiciously lacking) or some useful blood/tissue samples, the information vacuum will be ripe for speculation."

Some of the videos portray bodies that look very similar to images from Halabja in Iraqi Kurdistan, where Saddam Hussein killed at least 3,200 Kurds in 1988 in an hour-long poison gas attack, Bretton-Gordon said.

If the attack does prove to be a chemical weapons attack, then "the red line which seems to change shape and size has clearly been crossed," he said.

The Obama administration said Wednesday it is "deeply concerned" by the reports and urged the U.N. to investigate, according to a statement issued by Josh Earnest, deputy press secretary.

"We are working urgently to gather additional information," Earnest said. "The United States strongly condemns any and all use of chemical weapons. Those responsible for the use of chemical weapons must be held accountable. Today, we are formally requesting that the United Nations urgently investigate this new allegation."

Wednesday's attack looks like a Syrian government attack and should trigger a re-evaluation of the administration's Syria policy, said Michael Doran, who served as a deputy assistant secretary of Defense and a senior director at the National Security Council under President George W. Bush.

Assad has targeted civilians and killed thousands of women and children already, and the United States has determined that he used chemical weapons in the past, but U.S. leadership has been missing, Doran said.

"We should rethink our strategy and determine that ousting Assad is in the national interest of the United States," Doran said. " There needs to be more than words. There needs to be a strategy to oust Assad, to topple him."

Obama warned Assad on Aug. 20, 2012, "that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized." In June, the White House said U.S. intelligence had confirmed chemical weapons on a small scale used by the Syrian government against rebels, but the administration has hesitated to provide any significant military aid to the rebels.

To date, none of the lethal aid promised by the White House in June has arrived in rebel hands, said Khaled Saleh, chief spokesman for the Syrian Military Council of the Free Syrian Army, the coalition of rebel field commanders that the United States said it supports.

"What we want from the United States is to take a more active position," Saleh said. "We're trying to bring about a solution to a guy that is killing thousands of Syrians. If what Assad is doing today goes unpunished, it's a scary message to Assad that he has a green light to do what he's doing."